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Engine Control Module: Description and Operation

MODES OF OPERATION

As input signals to the Engine Control Unit (ECU) change, the ECU adjusts its response that it sends to the output devices. For example, the ECU must calculate a different injector pulse width and ignition timing for idle than it does for wide open throttle (WOT).

There are eight modes of operation that determine how the ECU responds to the various input signals. Modes of operation are separated into two different types, OPEN LOOP and CLOSED LOOP. During OPEN LOOP modes the ECU receives input signals and responds only to preset ECU programming. Input from the oxygen (02) sensor is not monitored during OPEN LOOP modes. During CLOSED LOOP modes the ECU monitors the oxygen sensor input. This input indicates to the ECU whether or not the calculated injector pulse width results in the ideal air/fuel ratio of 14.7 parts of air to 1 part of fuel. By monitoring the exhaust oxygen content, using the oxygen sensor, the ECU can "fine tune" the injector pulse width to achieve optimum fuel economy and performance combined with low emission levels.

The Multi-Point Fuel Injection (MPFI) system has the following modes of operation:

- Ignition switch ON
- Engine start-up (CRANK)
- Engine warm-up
- Idle
- Cruise
- Deceleration
- Wide Open Throttle (WOT)
- Ignition switch OFF

The ignition switch ON, engine start-up (CRANK), engine warm-up, deceleration, wide open throttle, and ignition switch OFF modes are OPEN LOOP modes. The idle and cruise modes, with the engine at operating temperature, are CLOSED LOOP modes.



IGNITION SWITCH ON

This is an OPEN LOOP mode. When the MPFI system is ACTIVATED by the ignition switch, the following actions occur:

- The system power relay is ENERGIZED for one to three seconds.
- The ECU stores a barometric pressure value, preparing for engine start-up.
- The ECU OPENS the Idle Speed Regulator (ISR).
- The fuel pump is ACTIVATED by the ECU.
(The fuel pump will operate for approximately one to three seconds unless the engine is operating or the starter motor is engaged).
- The power latch relay is ENERGIZED for 1 to 3 seconds.
- The oxygen sensor heater element is ACTIVATED by the ignition relay.
- The EGR valve solenoid is ENERGIZED by the ECU preventing EGR valve operation.
- The injectors are ENERGIZED by the fuel pump relay, but no ground is established by the ECU.

The following engine sensors are ACTIVATED and begin providing input to the ECU:

- Coolant temperature sensor
- Manifold air temperature (MAT) sensor
- Manifold absolute pressure (MAP) sensor
- Throttle position sensor (TPS)

Battery voltage is also measured when the ignition switch is turned ON.



ENGINE START-UP

This is an OPEN LOOP mode. The following actions occur when the starter motor is engaged. The ECU receives inputs from:

- Battery voltage
- Coolant temperature sensor
- Manifold air temperature (MAT) sensor
- Manifold absolute pressure (MAP) sensor
- Starter motor relay
- Speed sensor
- Ignition switch
- Throttle position sensor (TPS)
- Injectors (voltage present at injector)

The fuel pump is ACTIVATED by the ECU. Voltage is applied to the fuel injectors with the ECU controlling the injection sequence and injector pulse width. The ECU determines the proper ignition timing according to input received from the speed sensor. The EGR valve solenoid is ENERGIZED by the ECU preventing EGR operation. If the ECU detects a wide open throttle condition it assumes that a flooding condition exists. The ECU will then discontinue injector operation until engine speed reaches approximately 400 rpm.



ENGINE WARM-UP

This is an OPEN LOOP mode. During engine warm-up the ECU receives inputs from:

- Air conditioning select signal (if equipped)
- Air conditioning predict signal (if equipped)
- Battery voltage
- Coolant temperature sensor
- Manifold air temperature (MAT) sensor
- Manifold absolute pressure (MAP) sensor
- Neutral Safety Switch (Gear indicator signal-automatic transmission only)
- Speed sensor
- Ignition Switch
- Throttle position sensor (TPS)


Based on these inputs the following occurs:

- Voltage is applied to the fuel injectors with the ECU controlling the injection sequence and injector pulse width.
- The ECU adjusts engine idle speed through the idle speed regulator and ignition timing through the ignition control module.
- The ECU energizes the EGR valve solenoid to restrict EGR operation, and, if a request signal is received from the A/C Control Module (ACM), the ECU will adjust idle speed to compensate for the additional engine load.
- When the engine has reached operating temperature the ECU will begin monitoring the oxygen sensor input, the system will leave the warm-up mode and go into CLOSED loop operation.



IDLE

This is a CLOSED LOOP mode. At idle speed with the engine at operating temperature, the ECU receives inputs from:

- Air conditioning select signal (if equipped)
- Air conditioning predict signal (if equipped)
- Battery voltage
- Coolant temperature sensor
- Manifold air temperature (MAT) sensor
- Manifold absolute pressure (MAP) sensor
- Neutral Safety Switch (Gear-indicator signal automatic transmission only)
- Oxygen sensor
- Speed sensor
- Throttle position sensor (TPS)
- Ignition switch
- Injectors (voltage present at injector)

Based on these inputs the following occurs:

- Voltage is applied to the fuel injectors with the ECU controlling the injection sequence and injector pulse width (the length of injection time). The ECU monitors the 02 sensor input and uses that information to adjust air/fuel ratio.
- The ECU adjusts the air/fuel ratio by controlling the injector pulse width, adjusts the engine idle speed by controlling the idle speed regulator, and adjusts the ignition timing through the ignition control module.
- The ECU energizes the EGR valve solenoid to restrict EGR operation.



CRUISE

This is a CLOSED LOOP mode. At cruising speed with the engine at operating temperature, the ECU receives inputs from:

- Air conditioning select signal (if equipped)
- Air conditioning request signal (if equipped)
- Battery voltage
- Coolant temperature sensor
- Gear indicator signal (auto. trans. only)
- Manifold air temperature (MAT) sensor
- Manifold absolute pressure (MAP) sensor
- Oxygen sensor
- Speed sensor
- Throttle position sensor (TPS)

Based on these inputs the following occurs:

- Voltage is applied to the fuel injectors with the ECU controlling the injection sequence and injector pulse width (the length of injection time). The ECU monitors the 02 sensor input and uses that information to adjust air/fuel ratio.
- The ECU adjusts the air/fuel ratio by controlling the injector pulse width, adjusts the engine idle speed by controlling the idle speed regulator, and adjusts the ignition timing through the ignition control module.
- The ECU de-energizes the EGR solenoid. This allows manifold vacuum to reach the EGR valve transducer. If there is sufficient exhaust backpressure to CLOSE the transducer, EGR flow results.


DECELERATION

This is an OPEN LOOP mode. During deceleration the ECU ignores oxygen sensor input and receives the following inputs:

- Air conditioning select signal (if equipped)
- Air conditioning predict signal (if equipped)
- Battery voltage
- Coolant temperature sensor
- Manifold air temperature (MAT) sensor
- Manifold absolute pressure (MAP) sensor
- Speed sensor
- Throttle position sensor (TPS)

If the vehicle is under hard deceleration with the proper rpm and closed throttle conditions, the ECU will enter a fuel cut-off strategy in which it will not fire the injectors (this is done to ensure that an exhaust back-fire doesn't occur, and that emission levels are kept down). Once the hard deceleration conditions cease, the ECU will continue to fire the injectors. The ECU will also energize the EGR solenoid, preventing EGR operation.
If the ECU does not detect a hard deceleration condition, it will adjust injector pulse width based on engine control sensor inputs.



WIDE OPEN THROTTLE

This is an OPEN LOOP mode. During wide open throttle operation the ECU receives the following inputs:

- Battery voltage
- Coolant temperature sensor
- Manifold air temperature (MAT) sensor
- Manifold absolute pressure (MAP) sensor
- Throttle position sensor (TPS)
- Speed sensor
- Ignition switch
- Injectors (voltage present at injector)


During wide open throttle conditions the following occurs:

- The ECU monitors barometric pressure via the MAP sensor.
- Voltage is applied to the fuel injectors with the ECU controlling the injection sequence and injector pulse width.
- The ECU ignores the oxygen sensor input signal.
- The ECU adjusts ignition timing through the ignition control module, energizes the EGR valve solenoid to restrict EGR operation, and disengages the A/C compressor clutch through the clutch relay for approximately 15 seconds.





IGNITION SWITCH OFF

When the ignition switch is turned OFF, the following functions occur:

- The ECU stops energizing the injectors.
- Ignition system power is shut OFF.
- Fuel pump relay ground path is broken by the ECU which causes the fuel pump operation to cease.
- The ECU de-energizes the power latch relay, preventing voltage flow to the injection circuit.
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